Spawning objects with a fixed velocity in 3D?

I wanted to make a sort of flappy bird clone, but in 3D with objects like cube or cylinder instead of sprites.

I put in this code for the pipe object :

public class obstacle : MonoBehaviour {

 public Vector2 velocity = new Vector2 (-4, 0);
 void Start () {
     GetComponent<Rigidbody>().velocity = velocity;
 
 }
 

}

And this one for it's prefab:

public class generate : MonoBehaviour { public GameObject pipe;

 void Start () {
     InvokeRepeating ("CreateObstacle", 1f, 3f);
 }

 void CreateObstacle(){
     Instantiate (pipe);
 }

}

It seems to work alright in 2D with sprites, but breaks down in 3D. Object is instantiated, but does not move at all.

Any way to move them in a set direction with a fixed velocity?